This invention relates to a charging apparatus for an electrostatic copying machine.
In a typical copying machine, a photoconductive member such as a photoconductive drum is uniformly charged and then exposed with a light image corresponding to an original document to form an electrostatic image on the photoconductive member. A developer is applied to the drum to develop the electrostatic image into a visible toner image which is transferred and fixed to a sheet of paper.
It is well known that the charge acceptance of the photoconductive member in an electrostatic copying machine changes with temperature. Unfortunately, as the charge acceptance varies with temperature, the quality of the copy image or reproduction is affected.
For example, FIG. 1 shows a graph of charge acceptance versus temperature for a photoconductive member under constant luminance. The data in this graph was obtained by charging a photoconductive member formed by an alloy of selenium and tellurium with a corona charger under constant voltage. The ordinate shows the charge acceptance (V) and the abscissa shows the temperature (T.degree. C.) of the atmosphere surrounding the photoconductive member. The shaded portion is the permissible charge acceptance range for quality reproductions. It can be seen from this graph that, above 35.degree. C., the charge acceptance (V) decreases with temperature. Moreover, temperatures above 45.degree. C. are outside the permissible charge acceptance range. In other words, with temperature above 45.degree. C. and constant luminance, excessive exposure occurs which results in poor reproduction quality. This occurs because the electrical resistance of the photoconductive member is reduced with an increase in temperature which reduces the charge acceptance of the photoconductive member. Accordingly, it is necessary to control the charging of the photoconductive member by the corona charger in order to enhance reproduction quality.
In U.S. Pat. No. 3,805,069 issued to Donald H. Fisher on Apr. 16, 1974, a charging apparatus is disclosed which contains a thermistor for detecting the temperature of the photoconductive member and generating a signal representing the detected temperature. This signal is utilized to control the high voltage to the corona charger. However, such prior art charging apparatus have many disadvantages including unreliability, complicated construction and high cost.